This invention relates to an electroless gold plating bath, an electroless gold plating method using same, and electronic parts subjected to electroless gold plating by the method.
Gold exhibits the smallest ionization tendency among metals, meaning the most stable and most corrosion-resistant metal. In addition thereto, gold is excellent in electric conductivity and thus, has been in wide use in the fields of electronic industries. Immersion gold plating has been widely employed as a final surface treatment such as of circuits of printed board substrates and mounted or terminal portions of IC packages. In particular, the following methods are, for example, known with the following features, respectively.
(1) ENIG (Electroless Nickel Immersion Gold: Electroless Nickel/Immersion Gold)
                A method of forming an immersion gold plated coating formed on an underlying electroless nickel plated coating.        Capable of preventing diffusion of Co, preventing oxidation of nickel, and improving a corrosion resistance of circuits or terminals.        Usable for solder bonding.        Usable in wire bonding by forming thickened gold after treatment of ENIG.        With wire bonding, heat treatment is carried out after plating whereby nickel is diffused over a gold coating. To avoid this, electroless gold plating is performed on the nickel/immersion gold coating to increase the thickness of gold thereby coping with the diffusion of the nickel.(2) DIG (Direct Immersion Gold: Direct Immersion Gold)        A method of directly forming, on copper, an immersion gold plated coating.        Capable of preventing oxidation of copper, preventing diffusion of copper and improving a corrosion resistance of circuits and terminals.        Usable in solder bonding and wire bonding.        Well usable under conditions where a thermal load is not imposed appreciably (i.e. under conditions of a low thermal treating temperature, a reduced number of reflow cycles and the like) although long-term reliability is slightly inferior to that of nickel/gold, nickel/palladium/gold or the like.        Low in cost because of its simple process.(3) ENEPIG (Electroless Nickel Electroless Palladium Immersion Gold: Electroless Nickel/Electroless Palladium/Immersion Gold)        A method of forming an electroless palladium plated coating between an underlying electroless nickel plated coating and an immersion gold plated coating.        Capable of preventing diffusion of copper, preventing oxidation and diffusion of nickel, and improving a corrosion resistance of circuits and terminals.        Most suited for lead-free solder bonding which has been recently facilitated (because a lead-free solder needs a greater thermal load upon solder bonding than a tin-lead eutectic solder, and with nickel/gold, the bonding characteristic lowers).        Suited for wire bonding        No diffusion of nickel takes place if a gold thickness is not great.        Suited for the case where better reliability is obtained although nickel/gold is applicable.        
The immersion gold plating is such that gold is deposited by utilizing, in a plating bath, a difference in redox potential from an underlying layer such as of nickel, for which gold corrodes nickel to cause corrosion spots to occur owing to the oxidation (elution). The corrosion spots caused by the oxidation serve as an inhibition factor when tin and nickel in the solder layer are connected upon subsequent reflow of the solder, with the attendant problem that bonding characteristics such as strength lower.
In order to solve the problem, there have been disclosed an electroless gold plating bath including a sulfite adduct of aldehyde in Japanese Patent Laid-open No. 2004-137589 and a gold plating bath including a hydroxyalkylsulfonic acid in PCT Patent Publication No. WO 2004/111287, respectively. These techniques have for their object the suppression of corrosion of an underlying metal.
However, since these electroless gold plating baths have, as a reducing agent, a sulfonic acid group or a sulfite component, the following disadvantages are involved inherently to the case using a sulfonic acid group or sulfite component.
(1) Lowering of Deposition Rate
The sulfonic acid group or sulfite component acts as a stabilizer for gold deposition, thereby lowering a deposition rate of gold.
(2) Instability of Deposition Rate
A great difficulty is involved in control of a sulfonic acid group or sulfite component, thus leading to a difficulty in obtaining a stable deposition rate.
(3) Failure in Appearance in a Thickened State
Where thickening (0.1 μm or over) is performed using an electroless gold plating bath containing a sulfite component, the coating becomes reddish in appearance. This is because of deposition of particulate gold.
When using a primary amine compound where an amino group (—NH2) exists, such as triethylenetetramine, as described in the PCT Patent Publication No. WO 2004/111287, intergranular corrosion proceeds in the nickel surface thereby lowering the coverage of gold, with the attendant disadvantage that the resulting coating becomes red in appearance.